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D R Ort

Publications and source records attributed to D R Ort.

43 records · Page 3Linked to original sources
PubMed1974

Studies on the Energy-coupling Sites of Photophosphorylation: V. Phosphorylation Efficiencies (P/e(2)) Associated with Aerobic Photooxidation of Artificial Electron Donors.

The rate of Hill reaction can be measured accurately as O(2) uptake (the Mehler reaction) if a rapidly autoxidizable electron acceptor (e.g., methylviologen) is used. However, when an artificial electron donor-ascorbate couple (or ascorbate alone) replaces the natural donor, water, the rate of O(2) consumption is no longer a reliable measure of the electron flux, because superoxide radical reactions contribute to O(2) uptake. Such radical reactions, however, can be suppressed by adding enough superoxide dismutase to the reaction mixture. Indeed in all of the photosystem I- and photosystem II-donor reactions tested (except with benzidine which was tested without ascorbate added), the O(2) uptake was inhibited by 30 to 50% by the addition of superoxide dismutase. The rate of phosphorylation was totally unaffected by the enzyme. The reasessment of the phosphorylation efficiencies thus made by the use of superoxide dismutase led us to the following conclusions. The phosphorylation efficiency associated with the transfer of electrons from a donor to methlylviologen (than to O(2)) through both photosystems II and I is practically independent of the donor used-catechol, benzidine, p-aminophenol, dicyanohydroquinone, or water. The P/e(2) ratio is 1.0 +/- 0.1. Only ascorbate gives a slightly lower value (P/e(2) = 0.9). (NH(2)OH-treated, non-water-splitting chloroplasts were used for reactions with these artificial donors.) The phosphorylation efficiency associated with DCMU-insensitive, photosystem I-mediated transfer of electrons from a donor to methylviologen (then to O(2)) is again largely independent of the donor used, such as diaminodurene, diaminotoluene, and reduced 2,6-dichlorphenol-indophenol. The P/e(2) ratio is 0.6 +/- 0.08.

Journal Article↗
PubMed1973

Studies on the Energy-coupling Sites of Photophosphorylation: II. Treatment of Chloroplasts with NH(2)OH Plus Ethylenediaminetetraacetate to Inhibit Water Oxidation while Maintaining Energy-coupling Efficiencies.

Artificial electron donors to photosystem II provide an important means for characterizing the newly discovered site of energy coupling near photosystem II. However, water oxidation must be completely abolished, without harming the phosphorylation mechanism, for these donor reactions and the associated phosphorylation to withstand rigorous quantitative analysis. In this paper we have demonstrated that treatment of chloroplasts with hydroxylamine plus EDTA at pH 7.5 in the presence of Mg(2+) followed by washing to remove the amine is a highly reliable technique for this purpose. The decline of the Hill reaction and the coupled phosphorylation during the treatment were carefully followed. No change in the efficiency of phosphorylation (P/e(2) 1.0-1.1) was observed until the reactions became immeasurable. Photosystem I-dependent reactions, such as the transfer of electrons from diaminodurene or reduced 2,6-dichlorophenolindophenol to methylviologen, and the associated phosphorylation were totally unaffected. It is clear that the hydroxylamine treatment is highly specific, with no adverse effect on the mechanism of phosphorylation itself. Benzidine photooxidation via both photosystems II and I in hydroxylamine-treated chloroplasts (electron acceptor, methylviologen; assayed as O(2) uptake) supports phosphorylation with the same efficiency as that observed for the normal Hill reaction (P/e(2) = 1.1). An apparent P/e(2) ratio of 0.6 was computed for the photooxidation of ascorbate.

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